1. Field of the Invention
The present invention relates to a pigment with a specific tetraazaporphyrin skeleton, which serves as a charge generation material capable of generating charge carriers when exposed to light, and an electrophotographic photoconductor comprising a photoconductive layer which contains the above-mentioned pigment therein.
2. Discussion of Background
Conventionally, inorganic materials such as selenium, cadmium sulfide and zinc oxide are used as photoconductive materials of an electrophotographic photoconductor in the electrophotographic process. The above-mentioned electrophotographic process is one of the image forming processes, through which the surface of the photoconductor is charged uniformly in the dark to a predetermined polarity, for instance, by corona charge. The uniformly charged photoconductor is exposed to a light image to selectively dissipate the electric charge or the exposed areas, so that a latent electrostatic image is formed on the photoconductor. The thus formed latent electrostatic image is developed into a visible image by a toner comprising a coloring agent such as a dye or pigment, and a binder agent such as a polymeric material.
Fundamental characteristics required for the photoconductor in such an electrophotographic process are: (1) chargeability to an appropriate potential in the dark, (2) minimum dissipation of electric charge in the dark, and (3) rapid dissipation of electric charge when exposed to light.
However, while the above-mentioned inorganic materials have many advantages, they have several shortcomings from the viewpoint of practical use.
For instance, a selenium photoconductor has the shortcomings that the manufacturing conditions are difficult and, accordingly, its production cost is high. In addition, it is difficult to work it into the form of a belt due to its poor flexibility, and it is so vulnerable to heat and mechanical shocks that it must be handled with the utmost care.
A cadmium sulfide photoconductor and a zinc oxide photoconductor can be easily obtained by dispersing cadmium sulfide particles and zinc oxide particles respectively in a binder resin, and coating the thus prepared coating liquid on a support. However, they are poor in terms of the mechanical properties, such as surface smoothness, hardness, tensile strength and wear resistance. Therefore, they cannot be used in the repeated operations as they are.
To solve the problems of the inorganic materials, various electrophotographic photoconductors employing organic materials are proposed in recent years and some are still put to practical use. For example, there are known a photoconductor comprising poly-N-vinylcarbazole and 2,4,7-trinitrofluorene-9-on, as disclosed in U.S. Pat. No. 3,484,237; a photoconductor prepared by sensitizing poly-N-vinylcarbazole with a pigment of pyrylium salt, as disclosed in Japanese Patent Publication 48-25658; a photoconductor comprising as the main component an organic pigment as disclosed in Japanese Laid-Open Patent Application 47-37543; and a photoconductor comprising as the main component a eutectic crystal complex of a dye and a resin, as disclosed in Japanese Laid-Open Patent Application 47-10735.
In particular, a laminated-type photoconductor fabricated by successively overlaying a charge generation layer in the form of a thin film of an organic pigment and a charge transport layer comprising a charge transport material on an electroconductive support has been actively studied because the sensitivity of the photoconductor is high and there are a large variety of materials therefore Thus, the laminated-type photoconductor has become the mainstream in the field of the copying machine and printer. However, the conventional laminated-type photoconductors are still unsatisfactory in light of such requirements for the advanced photoconductor as to cope with high speed operation of the copying machine and show high sensitivity in the wavelength range of the semiconductor laser.
In recent years, the copying machine is required not only to produce high quality images, but also to be provided with text editing function and composite processing function. In line with the above-mentioned demands, non-impact printing technology has been developed and digital recording apparatuses such as a laser printer, laser facsimile machine and digital copying machine have been widely utilized.
Most of the above-mentioned digital recording apparatuses employ as a light source a semiconductor laser beam because it is compact, cheap and convenient. The wavelength of the currently used semiconductor laser beam is limited to 600 nm or more, so that the electrophotographic photoconductors used in the above-mentioned digital recording apparatuses are required to show sufficient photosensitivity in the wavelength range of at least 600 to 850 nm.
The organic photoconductive materials, for example, a phthalocyanine pigment, azo pigment, cyanine pigment, azulene pigment, and squarylium pigment are conventionally known to satisfy the above-mentioned requirements. In particular, the phthalocyanine pigment can show absorption and photosensitivity in the relatively long wavelength range. In addition, a variety of phthalocyanine pigments can be obtained according to the kind of central metal or the type of crystalline form. Therefore, research and development of the phthalocyanine pigment has been actively conducted to obtain the improved electrophotographic photoconductor for use with the semiconductor laser.
There are conventionally known .epsilon.-type copper phthalocyanine, X-type metal-free phthalocyanine, .tau.-type metal-free phthalocyanine, vanadyl phthalocyanine and titanyloxy phthalocyanine (Japanese Laid-Open Patent Applications 8-231869, 8-66595 and 8-13942). However, any of the above-mentioned phthalocyanine compounds are still insufficient in terms of photo-sensitivity, chargeability, and the durability in the repeated use.